This invention relates to the treatment of industrial waste waters containing chromium, e.g., in the form of chromic acid, metallic chromate salts, etc. Such chromic materials can no longer be discharged directly into sewers, rivers or streams due to various legal requirements imposed by the environmental authorities. Also, chromium is an expensive metal and the treating solutions and salts are desirably recovered from the economic point of view. An economical process for not only removing chromium from waste waters but also recovering it efficiently has long been desired.
One prior-art method for purifying industrial waste waters containing chromium materials evolves about reducing the chromium, viz., chromic ion, with sulfur dioxide, sodium bisulfite, or like materials, followed by a subsequent precipitation of the chromium with a basic substance such as lime (calcium hydroxide) or similar material. The precipitated chromium is then filtered.
Another prior-art process involves the precipitation of chromium with barium in acidic aqueous media utilizing a strong acid followed by filtration to remove the precipitated chromium. The latter procedure, involving direct precipitation of chromium, is not only more expensive than the sulfur dioxide reduction-lime precipitation process, but also adds the additional problem of removal of barium from the waste waters prior to discharge. Such prior art barium precipitation treatment procedures result in unacceptably high concentration of barium waste the wate water necessitating removal thereof prior to discharge into sewers, rivers or streams. U.S. Pat. No. 3,371,034 to Richards illustrates a direct precipitation procedure utilizing barium carbonate in aqueous solutions acidified with strong acids such as nitric or hydrochloric acid or their salts. Such prior art procedures also encounter difficulties in separation of the chromium solids from the liquid waste media necessitating the use of one or more settling tanks, the use of which is avoided in accordance with the process of this invention.